Curable prepregs with surface openings

ABSTRACT

Curable prepregs possessing enhanced ability for the removal of gases from within prepregs and between prepreg plies in a prepreg layup prior to and/or during consolidation and curing. Each curable prepreg is a resin-impregnated, woven fabric that has been subjected to a treatment to create an array of openings in at least one major surface. Furthermore, the location of the openings is specific to the weave pattern of the fabric.

BACKGROUND

Fiber-reinforced polymer composites are high-performance structuralmaterials that are composed of a resin matrix and reinforcement fibers.These fiber-reinforced polymer composites have been used for fabricatingstructural parts that require high strength, and/or low weight, andresistance to aggressive environments. Examples of such structural partsinclude aircraft components (e.g. tails, wings, fuselages, propellers).The fibers reinforce the matrix resin, bearing the majority of the loadsupported by the composite, while the resin matrix bears a minorityportion of the load supported by the composite and also transfers loadfrom broken fibers to intact fibers. In this manner, these polymericcomposites may support greater loads than either the matrix resin orfibers may support alone. Furthermore, by tailoring the reinforcingfibers in a particular geometry or orientation, the composite can beefficiently designed to minimize weight and volume.

Fiber-reinforced polymer composites are traditionally made from sheetsof resin-impregnated fibers, also known as prepregs. To form a compositepart from the prepregs, a plurality of prepreg layers may be laid upwithin a mold, and heat may be applied to cause the matrix resin toflow, enabling consolidation of the prepreg layers. The applied heat mayadditionally cure or polymerize the matrix component.

The consolidation of prepregs to form composites in this manner presentsproblems, however. Gases such as air may be trapped inside theindividual prepreg and between the prepreg layers during layup.Furthermore, volatiles may also evolve during heating and/or curing ofthe prepregs. These gases are difficult to remove from the layup, as thematrix substantially inhibits movement of the gases and may result inporosity within the final, cured composite. Porosity refers to the voidswithin the cured composite material. This porosity could furthernegatively affect the mechanical properties of the final, curedcomposite.

Techniques have been developed to enhance removal of entrapped gasesduring composite fabrication, however, problems remain. For example,edge breathers may be employed to apply vacuum to the edge of prepregsin order to draw out gases from the sides of prepreg layers. However,removal of trapped gases from prepregs in this manner is slow and maynot substantially remove all of the trapped gases.

SUMMARY

Disclosed herein are curable prepregs possessing enhanced ability forthe removal of gases from within prepregs and between prepreg plies in aprepreg layup prior to and/or during consolidation and curing. Eachcurable prepreg is a resin-impregnated, woven fabric that has beentreated to create an array of openings in at least one major surface.Furthermore, the location of the openings is specific to the weavepattern of the fabric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a woven fabric portion where fibers towin one weaving direction passes over and then under tows in thetransverse direction.

FIG. 2 schematically shows a partially impregnated fabric according toan embodiment of the present disclosure.

FIG. 3 schematically shows openings created in one surface of a prepregaccording to an embodiment of the present disclosure.

FIG. 4 schematically shows openings created in opposite surfaces of aprepreg according to an embodiment of the present disclosure.

FIG. 5 schematically shows a partially impregnated prepreg according toan embodiment.

FIG. 6 shows the prepreg of FIG. 5 after thermal treatment.

FIG. 7 schematically shows a portion of a satin weave fabric.

FIG. 8 schematically shows a resin surface with surface openings formedon the satin weave fabric of FIG. 7.

FIG. 9 schematically shows a partially impregnated prepreg that has beensubjected to thermal treatment to create surface openings according toanother embodiment.

FIG. 10 schematically shows a portion of a plain weave fabric.

FIG. 11 schematically shows a resin surface with surface openings formedon the plain weave fabric.

FIG. 12 illustrates an exemplary prepregging system that is capable ofmanufacturing a prepreg fabric.

FIG. 13 schematically shows a configuration for assembling a honeycombcore sandwich structure according to an example.

FIG. 14 schematically shows the honeycomb core sandwich structureproduced from the assembly shown in FIG. 13.

FIGS. 15-17 are micrographs showing a top-view image of a heat-treatedprepreg surface at 1 minute, 4 minutes, and 7.5 minutes, respectively,wherein the prepreg was produced using a satin weave fabric according toone example.

FIG. 18 is a micrograph showing the top-view image of a heat-treatedprepreg surface, wherein the prepreg was produced using a plain weavefabric according to another example.

FIG. 19 is a micrograph showing the top-view image of a cured prepregsurface with air bubbles formed beneath the surface.

FIG. 20 shows the cross-section view of a cured composite panelconsisted of untreated prepreg material.

FIG. 21 shows the cross-section view of a cured composite panelconsisted of heat-treated prepreg material.

DETAILED DESCRIPTION

The curable prepreg disclosed herein is composed of a resin-impregnatedwoven fabric. The woven fabric has two opposing faces and a weavingpattern in which one or more tows in a first weaving direction floatover one or more tows in a second weaving direction, then pass under oneor more tows in the second weaving direction, wherein a crossover/underlocation on a face of the fabric is created when a first tow in thefirst weaving direction crosses over a second tow in the second weavingdirection then passes under an adjacent third tow in the second weavingdirection, or when the first tow passes under a second tow in the secondweaving direction then crossover an adjacent third tow in the secondweaving direction. The crossover/under location in this context refersto the portion of the first tow that is going up or going down betweenthe adjacent second and third tows.

The woven fabric for producing the prepreg is made from fiber tows. Thetows are interlaced in a weaving pattern in which one or more tows in afirst weaving direction float over one or more tows in a second weavingdirection, then pass under one or more tows in the same second weavingdirection. Due to the weaving configuration, the two major faces of thefabric contain pockets therein, thus, they are neither smooth nor flatthroughout.

FIG. 1 schematically illustrates that, due to the weaving configurationof the woven fabric, there are pockets P created in the fabric surfacewhenever there is a tow portion which crosses over or passes underanother transverse tow, i.e., the crossover/under location. Stillreferring to FIG. 1, going from left to right, when a tow 11 in thefirst weaving direction crosses over another tow 12 in asecond/transverse weaving direction then passes under an adjacent tow 13in the same second/transverse weaving direction, a “down” tow portion T₁is created, and when tow 11 passes under tow 13 then over an adjacenttow 14 in the second/transverse weaving direction, an “up” tow portionT₂ is created. These “up” and “down” tow portions result in the pocketsP. In other words, if the fabric is lying on a horizontal, planarsurface, the pocket P is created whenever there is a change in elevationof the tow relative to the planar surface. It should be understood thatFIG. 1 shows only one example of a fabric weave, and that more complexfabric weaves such as tri-axial weave are contemplated herein.

The curable prepreg further has a curable, hot-melt resin film coveringeach face of the fabric and penetrates partially through the thicknessof the fabric, leaving a middle section of the fabric, in the thicknessdirection, substantially free of the resin film. An array of openings isformed in one or both of the resin films, wherein each opening exposesthe pocket (P in FIG. 1) formed at the crossover/under location in thefabric weave pattern, according to one embodiment. In some embodiments,the resin film is continuous everywhere except where the openings arelocated. According to another embodiment, the array of openings in theresin is aligned with the interstices in the fabric weave. Thisembodiment pertains to certain woven fabrics such as plain weave fabric.

The openings are configured to enable gases, such as air, to flow fromthe middle section of the fabric to at least one outer surface of theprepreg, or from at least one outer surface of the prepreg to the middlesection, or from one outer surface of the prepreg to an oppositesurface, or combination thereof. The openings also enable gas transferas well as establishment of vacuum which provides the driving force forthe resin to impregnate vacuum-rich areas.

The openings disclosed herein are specific to the fabric weave, unlikeprepregs with surface openings formed by conventional mechanicaltechniques, which are used to form global hole pattern or random holepattern.

For the purposes herein, the term “curable” means not fully cured, andincludes uncured condition.

Each tow is a bundle of fiber filaments. The number of filaments in eachbundle may be in multiple of 1000, e.g. 1000-75,000. Tows having lessthan 15,000 filaments per bundle are contemplated for the intendedpurposes disclosed herein. The term “filament” refers to a relativelyflexible, continuous structure having a high length-to-width ratio.

Fiber materials for the fiber tows include, but are not limited to,glass (including electrical or E-glass), carbon (including graphite),aramid (e.g. Kevlar), high-modulus polyethylene (PE), boron, quartz,basalt, ceramic, polyester, poly-p-phenylene-benzobisoxazole (PBO), andcombinations thereof. For producing high performance compositematerials, e.g. materials for aerospace application, fibers havingtensile strength of greater than 3500 MPa are desirable.

The weaving configuration of the fabric is not limited and may includeplain weave, satin weave, twill weave, and the like. In a roll offabric, the longitudinal tows are in the warp direction and the lateraltows are in the weft direction. In plain weave, the warp and weft towsform a simple criss-cross pattern. Each weft tow crosses the warp tow bygoing over one, then under the next, and so on. The satin weave ischaracterized by two or more weft tows passing over a single warp tow,or vice versa, two or more warp tows floating over a single weft tow.The twill weave is characterized by passing the weft tow over one ormore warp tow and then under one or more warp tows and so on, with anoffset between rows to create the characteristic diagonal pattern.

FIG. 2 schematically illustrates a partially impregnated fabric (i.e.prepreg ply or prepreg fabric) according to an embodiment of the presentdisclosure. A fabric with fiber tows 20 is covered on both major faceswith an upper resin film 21 and a lower resin film 22. Each resin filmpenetrates partially through the thickness (T_(f)) of the fabric,leaving a middle section of the fabric substantially free of the resin.After partial impregnation, a plurality of enclosed air pockets 23 isformed between the resin films and the fabric as illustrated in FIG. 2.The enclosed air pockets coincide with the pockets formed at thecrossover/under locations in the fabric weave.

The weight ratio of fabric-to-matrix resin in the curable, porousprepreg may be varied, as dictated by the application. In oneembodiment, the weight fraction of the fabric may range from 20 wt. % to80 wt. %, on the basis of the total weight of the prepreg. In anotherembodiment, the weight fraction of fabric in a porous prepreg is lessthan 20 wt. %, when the porous prepreg is used as a surfacing film on acomposite substrate or a prepreg layup. The fraction of the prepregoccupied by the matrix resin may also be varied as desired. In certainembodiments, the matrix resin may occupy between about 20 wt. % to 80wt. % of the prepreg on the basis of the total weight of the prepreg.

Prepregging Method

According to one embodiment, the method for making the curable prepregdisclosed above includes partially impregnating a woven fabric with acurable matrix resin followed by a thermal treatment to create an arrayof surface openings. The method for partially impregnating the wovenfabric with a matrix resin is not limited, but a “hot-melt” prepreggingmethod is preferred. In general, this prepregging method ischaracterized by impregnating a fabric ply with a hot-melt resincomposition, in molten form, to yield a partially impregnated prepreg.Impregnation may be done by sandwiching a fabric ply between two resinfilms and pressing the obtained laminate by hot plates, heated rollers,or by a method in which the laminate is pressed between hot metal belts.Alternatively, the fabric is laminated to a resin film on one side only,leaving the other side substantially free of resin.

As an example, a curable, hot-melt resin composition may be applied inthe form of a thin resin film onto a release paper, and the resultingresin film released therefrom is laminated and formed on a fabric ply.Heat is applied to lower the viscosity of the resin film so that it isin a molten state and can penetrate the fabric to a desired level,preferably only partially through the fabric in order to leave a centraldry portion. It should be understood that the elevated temperatureapplied during impregnation is lower than the onset curing temperatureof the hot-melt resin. Sufficient pressure is also applied duringlamination so that the resin film penetrates partially through thethickness of the fabric ply, thereby resulting in the fabric ply beingpartially impregnated with the resin composition in the thicknessdirection. The matrix resin remains uncured immediately afterimpregnation. For some embodiments, the resin film applied on each faceof the fabric may have a film weight of 10-200 gsm (g/cm²), and thefabric may have a fabric areal weight (FAW) of 100-600 gsm. Afterpartial impregnation, a continuous resin film covers one or both majorfaces of the fabric and penetrates partially through the thickness ofthe fabric, leaving the middle section of the fabric substantially freeof the resin.

During the thermal treatment, a release or backing paper is left on theexposed surface of the prepreg resin film and heat is applied until theresin film becomes flowable. In certain embodiments, the resin viscosityduring thermal treatment is less than 500 Poise at 90° C. Heating iscarried out until the resin film portions over the air pockets breakopen, creating openings in the resin film that correspond to locationsof the air pockets. The resin film breaks up by dewetting from therelease film surface with resin moving laterally towards areas adjacentto the pocket. In some cases, the resin film breaks away at the edges ofthe air bubble and moves laterally inward, leaving a minor drop of resinthat is removed when the release paper is peeled from the prepreg. Theopenings may be created in one surface of the prepreg as illustrated inFIG. 3 (openings 30), or in both opposing surfaces of the prepreg asillustrated in FIG. 4 (openings 40). As a result of the thermaltreatment, the openings create fluid passages for transporting air orother gases from the outer surface(s) of the prepreg to the middlesection of the fabric.

The thermal treatment for creating openings in the prepreg may becarried out as a post-treatment after the fabric has been partiallyimpregnated with the resin films by a standard prepregging process.Alternatively, the thermal treatment may be done in-situ during theprepregging process. It should be understood that the elevatedtemperature applied during thermal treatment is lower than the onsetcuring temperature of the matrix resin, and is used to initiate the flowof resin in order to open the enclosed air pockets.

In one embodiment, a resin-impregnated satin weave fabric is subjectedto a post-treatment to create an array of openings. Referring to FIG. 5,a satin weave fabric 50 is sandwiched between an upper resin film 51 anda lower resin film 52. The upper resin film 51 is formed on a releasepaper 53 and the lower resin film 52 is formed on a release (or backing)paper 54. The release paper may be coated with a silicone film. Theresulting laminate is subjected to hot-pressing to form a partiallyimpregnated prepreg, for example, in a prepregging machine. After resinimpregnation, air pockets 55 are created below the resin film. Next,referring to FIG. 6, the release paper 54 that is attached to the lowerresin film 52 is replaced with a polyester film 56. The polyester filmis placed on one side after removal of one of the release papers tofacilitate rolling up the final prepreg. Still referring to FIG. 6, thepartially impregnated prepreg with the release paper and polyester filmthereon is then heated in a heating cycle whereby the resin portions ofthe upper resin film 52 that lie over the air pockets break open andmove/flow away from the air pockets. As a result, openings 57 arecreated in the resin film 51. Heat treatment may be carried out byexposing the prepreg to a heat source, which is set to a pre-determinedtemperature, for a selected time period. The prepreg may be stationaryduring heat exposure, or may be moving through a heating zone via acontinuous process. Alternatively, other heating sources may be employedsuch as Hot Plate, Laser, heated drum, ultrasonic, hot air jet, etc.).The temperature and time period of the heat treatment may be varieddepending on the minimum resin viscosity to enable flow and thesufficient time for flow to occur. Thinner resin films flow faster, thushigher film weights require more time. As an example, the post-treatmentmay be applied to a roll of prepreg material, in the form of acontinuous sheet that has been formed via a standard continuous prepregmanufacturing method. In such post-treatment, the continuous prepreg isunwound and continuously conveyed, under tension, through a horizontalheating oven where the prepreg is exposed to heat, and then wound uponto a take-up roll. Depending on the length of the oven, the heatexposure time versus the conveying speed can be controlled to create thedesired openings. In one embodiment, the impregnating resin is a hotmelt epoxy-based matrix that is a viscoelastic solid at room temperature(20° C.-25° C.) and is curable within the temperature range of 250°F.-350° F. (121° C.-177° C.); the heating cycle may be carried out for0.25-20 minutes within the range of 120° F.-250° F. (49° C.-121° C.).

FIG. 7 shows an exemplary satin weave configuration, more specifically,8-Harness satin weave, and the locations 70 where air pockets may becreated when the satin weave fabric is partially impregnated with theresin films as described above. It should be understood that, inreality, the intersecting tows shown in FIG. 7 are actually closertogether and more tightly woven. FIG. 8 schematically shows aheat-treated prepreg surface, after resin impregnation and heattreatment as described above in reference to FIGS. 5 and 6, and thelocations of the openings in the resin film relative to thecrossover/under locations in the fabric weave.

FIG. 9 schematically shows a partially impregnated plain weave fabric 95that has been subjected to thermal treatment to create openings 100,which are formed through the upper and lower resin films 96, 97. Theresin films 96, 97 are supported by release/backing papers 98, 99,respectively.

FIG. 10 shows an exemplary plain weave fabric and the interstices 101formed therein. It should be understood that, in reality, theintersecting tows shown in FIG. 7 are actually closer together and moretightly woven. FIG. 11 schematically shows a heat-treated prepregsurface, after resin impregnation and heat treatment as described abovein reference to FIG. 9, and the locations of the openings in the resinfilm relative to the interstices 101 in the fabric weave. Note that theopenings are aligned with the interstices 11.

FIG. 12 schematically shows an exemplary prepregging system that iscapable of manufacturing a prepreg fabric and providing an in-situthermal treatment. Referring to FIG. 12, a continuous fabric web 80 isconveyed to a first pressure nip formed by a pair of heated pressurerollers 81, 82. The fabric web 80 is sandwiched between two resin films83, 84, which are unwound from supply rollers 85, 86. Each of the resinfilms 83, 84 is formed on a continuous release paper. The resin films83, 84 are pressed onto the top and bottom faces, respectively, of thefabric web 80 with the aid of pressure rollers 81, 82. Pressure and heatfrom the pressure rollers 81, 82 causes the resin films 83, 84 topartially impregnate the fabric web 80, thereby forming a partiallyimpregnated prepreg. The partially impregnated prepreg then passes overa heating plate 87. At this point, heating is carried out to createopenings in the prepreg. After thermal treatment, the resulting porousprepreg is conveyed over a cooling plate 88, where the porous prepreg iscooled to solidify the resin. The cooled prepreg is then conveyed bypull rollers 89, 90 and guided by additional guide rollers to a windingroll 91 where it is wound up. This type of process is particularlysuitable for creating openings in a prepreg that is based on the use ofa plain weave fabric, particularly, low GSM fabrics where the thermaltreatment is rapid due to thinness of fabric and impregnating film(s).

The openings formed in the heat-treated prepregs are irregular in shapeand are not uniform in sizes. The shapes and sizes of the openingsdepend on the weaving pattern and the thermal treatment time. The sizeof the openings increases as resin flow progresses with time. Asexamples, the openings may be substantially circular cross-section withdiameter within the range of 0.1-3 mm or approximately rectangular incross-section with width and length within the range of 0.1 mm-3 mm.Furthermore, after certain amount of treatment time, as example,treatment time of 1-8 minutes may be sufficient to create the openings.Moreover, after certain treatment time period, some openings may becomeconnected to each other, depending on the initial proximity of theopenings to each other. In some instances, some entrapped air pocketsmay not open due to imperfect processing conditions, for example, whenthe release paper is not adhering to the resin film during thermaltreatment.

Matrix Resin

The matrix resin for producing the curable prepreg described herein isbased on a curable hot-melt composition, characterized in that it isinitially a solid or semisolid at approximately room temperature (20°C.-25° C.), becomes molten at an elevated temperature at which thematerial is applied, solidified upon cooling, and is hardenable bycuring. Moreover, the matrix resin should have sufficient viscosity andwetting characteristics to allow the formation air pockets, andsubsequently, the formation of openings over the air pockets with heattreatment. In one embodiment, the hot-melt resin composition is acurable thermoset resin composition composed of one or more thermosetresins as the major component, and is substantially free of any organicsolvent such as acetone, methyl ethyl ketone (MEK), dioxolane, alcohol.When used to produce a finished cured product, these thermoset resinsare cured by the use of a catalyst or curing agent, heat or acombination of the two.

Suitable thermoset resins may include, but are not limited to, epoxies,unsaturated polyesters, bismaleimide, and combinations thereof. Thesethermoset resins can be fully cured by the use of heat, or a curingagent, or a combination thereof. Catalysts may be used to accelerate thecuring reaction. When thermoset resins are fully cured, they becomehardened and cannot be converted back to their original form.

In one embodiment, the matrix resin is an epoxy-based thermosetcomposition which contains one or more multifunctional epoxy resins asthe main polymeric component. Suitable epoxy resins include polyglycidylderivatives of aromatic diamine, aromatic mono primary amines,aminophenols, polyhydric phenols, polyhydric alcohols, polycarboxylicacids. Examples of suitable epoxy resins include polyglycidyl ethers ofthe bisphenols such as bisphenol A, bisphenol F, bisphenol S andbisphenol K; and polyglycidyl ethers of cresol and phenol basednovolacs.

Suitable bismaleimide resins may include N,N′bismaleimides of 1,2ethanediamine, 1,6-hexanediamine, trimethyl-1,6-hexanediamine,1,4-benzene-diamine, 4,4′-methylenebisbenzenamine,2-methyl-1,4-benzenediamine, 3,3′-methylenebisbenzenamine,3,3′sulfonylbisbenzenamine, 4,4′-sulfonyl-bisbenzenamine,3,3′oxybisbenzenamine, 4,4′oxybisbenzenamine,4,4′-methylenebiscyclohexanamine, 1,3-benzenedimethanamine,1,4-benzene-dimethanamine, and 4,4′-cyclohexanebisbenzenamine andmixtures thereof.

The matrix resin may further include, in minor amounts, thermoplasticmaterials such as polysulphones, polyether sulphones, polyether ketones(e.g. polyether ketone (PEK), polyether ether ketone (PEEK), polyetherketone ketone (PEKK) and the like), combinations thereof, and precursorsthereof. One or more thermoplastic materials are added to the matrixresin to increase the toughness, tackiness and drapability of theprepreg.

The matrix resin, as discussed herein, may further comprise additives,in minor amounts, to influence one or more of mechanical, rheological,electrical, optical, chemical, and/or thermal properties of the matrix.Such additives may further comprise materials that chemically react withthe matrix, interact with the matrix, or are unreactive with the matrix.Examples of additives may include, but are not limited to, tougheningparticles, flame retardants, ultraviolet (UV) stabilizers, antioxidants,colorants, and fillers (e.g., fumed silica, alumina, calcium carbonate,talc) to enhance one or more of damage tolerance, toughness, wearresistance.

The prepregs with surface openings (i.e. porous prepregs), as disclosedherein, are configured to enable dimensional stability of the openings.The openings, once formed, may remain dimensionally stable for aselected period of time. In certain embodiments, the openings may remaindimensionally stable during storage of the porous prepregs. Thedimensional stability may be provided by tailoring the viscosity of thematrix resin. The matrix resin is formulated to form dimensionallystable openings at about room temperature, but the resin is capable offlowing at an elevated temperature during consolidation or curing tofill out the openings.

“Full impregnation”, as discussed herein, refers to embeddingsubstantially all of the fabric fibers within the matrix resin. “Partialimpregnation”, as discussed herein, refers to impregnation that is lessthan full impregnation, whereby there are regions of dry fibers that arenot embedded within the matrix resin. In a preferred embodiment, thematrix resin is applied to both surfaces of the fabric ply, but theresin penetrates only partially through the thickness of the fabric soas leave a middle section of the fabric, in the thickness direction,substantially free of resin.

The term “dimensional stability” as used herein refers to the ability ofa structure to maintain dimension within a selected range for a selectedperiod of time. In certain embodiments, the selected range may bedetermined by the ability of the structure to perform an intendedfunction, such as allowing the passage of a gas at a selected rate undera selected pressure.

The term “room temperature” as used herein refers to temperatures withinthe range of 20° C. to 25° C.

Prepreg Layups and Composite Parts

To form a composite part, a plurality of curable prepregs disclosedherein may be laid up into a prepreg layup, and then the layup isconsolidated and cured. Consolidation and curing may be performed in asingle stage or separately.

It has been discovered that the prepregs with surface openings (i.e.porous prepregs) facilitate the removal of gases from individualprepregs and prepreg layup containing one or more porous prepregstherein prior to and/or during consolidation, and thus reduce the volumeof porosity within composites formed therefrom, as compared tocomposites formed without porous prepregs. For example, the openingsprovide escape routes for gases from within the porous prepregs andenable the gases to be removed from the prepregs with greater ease andin greater volume as compared with un-treated prepregs. The gases mayinclude gases that originate from within the matrix resin or resin-freezone of the partially impregnated prepreg, or gases that originate fromthe interlayer region between prepreg layers. In particular, the porousprepregs enable the removal of gases that may evolve from the resincomposition during consolidation.

The term “prepreg layup” as used herein refers to a plurality ofprepregs that are placed adjacent one another in a stacking arrangement.In certain embodiments, the prepregs within the layup may be positionedin a selected orientation with respect to one another. In a furtherembodiment, the prepregs may optionally be stitched together with athreading material in order to inhibit their relative motion from aselected orientation. In additional embodiments, “layups” may compriseany combination of fully impregnated prepregs, partially impregnatedprepregs, and porous prepregs as discussed herein. Layups may bemanufactured by techniques that may include, but are not limited to,hand layup, automated tape layup (ATL), automated fiber placement (AFP)and filament winding.

Consolidation refers to a process that takes place under the action ofone or more of heating, vacuuming, and applied pressure, whereby thematrix resin flows so as to displace void spaces. For example,consolidation may result in, but is not limited to, flow of resin intovoid spaces between fibers in the prepreg, void spaces between prepregs,and the like.

The terms “cure” and “curing” as used herein may include polymerizingand/or cross-linking processes. Curing may be performed by processesthat include, but are not limited to, heating, exposure to ultravioletlight, and exposure to radiation. In further embodiments, the matrixresin within the porous prepreg may be formulated or partially cured inorder to exhibit a selected stickiness or tack.

When a plurality of curable prepregs with surface openings are used in aprepreg layup, the layup possesses enhanced ability for the removal ofgases (e.g. air) trapped within the prepregs and between prepreg plies.During consolidation of the prepreg layup, the openings and theresin-free regions within the prepregs provide various routes for gasestrapped within the prepregs and between the prepregs to escape, therebyreducing the porosity within the resulting consolidated composite.Consequently, upon curing, the cured composite exhibits improvedmechanical properties. For example, cured composites having residualporosity of less than 1 vol. %, on the basis of the total volume of thecomposite, may be achieved in this manner.

When openings are formed in both major surfaces of the curable prepreg,gases may travel through the prepreg by entering one surface and exitingthrough the opposite surface. The openings also allow for vacuum tofully penetrate the laminate stack of prepregs. Moreover, the openingscreate channels with adjacent crossover/under locations to create airpassage along the interface of two adjacent prepreg plies. Various flowpaths may be created by any combination of surface openings, theinterlayer regions, and the non-impregnated (resin-free) portions of theprepregs. For example, gases from the interlayer region between adjacentprepregs may enter through openings on one side of a prepreg, and thenthrough the resin-free middle section of the same prepreg in order toescape to the outside. Alternatively, the gases may flow from oneinterlayer region to the next interlayer region via openings in oppositesides of each prepreg, and eventually flow out of the prepreg layup.This is an improvement as compared to standard products without thermaltreatment, because in a standard product where the resin film remainsintact, it is more difficult to get air to transfer from between pliesand into the ply core, whereas with the thermal-treated material, airtransfer is enhanced due to the number of openings. These various flowpaths greatly enhance the ability of entrapped gases to escape theprepreg layup and also create channels with adjacent up/down regions tocreate breathing along the interface of two adjacent plies.

The viscosity of the matrix resin may be controlled to flow and fillvoid spaces within and between the prepregs during consolidation. Forexample, in one embodiment, the viscosity of the matrix resin may becontrolled by resin formulation to flow and fill void spaces uponapplication of heat, without external pressure. In another embodiment,the viscosity of the matrix resin may be controlled by resin formulationto flow and fill void spaces upon application of heat and externalpressure, and optionally under vacuum. Beneficially, by allowing theopenings and other void spaces to be filled during consolidation, theporosity of the resultant composite is substantially reduced oreliminated.

A vacuum bag setup may be employed to perform consolidation of theprepreg layup. In this setup, the curable prepreg layup may be placed incontact with a tool and then enclosed with an impervious membrane. Thetool may have a relatively planar surface, curved surface, or otherthree-dimensional configuration. In one embodiment, a breather layer,such as an unimpregnated fiberglass sheet, may be positioned adjacent atleast one of the horizontal surfaces of the layup for surface breathing.Sealant tapes may be further used, as necessary, to create anapproximately vacuum tight seal between the tool and the membrane. Toinhibit flow of the resin outside of the layup, or to improve gas flow,one or more dams may also be placed adjacent the edges of the layup. Aperforated release film (e.g. perforated polyester film) may be insertedbetween the breather layer and the prepreg layup and a solid releasefilm (e.g. polyester film) may be inserted between the prepreg layup andthe tool in order to facilitate the removal of the consolidatedcomposite from the setup. The enclosed volume is evacuated and the layupis heated up slowly to cause consolidation. Heating may be applied byplacing the vacuum bag setup in an oven or an autoclave. Moreover,heating may be carried out with pressure (e.g. in an autoclave) orwithout pressure (e.g. within an oven), in order to lower the viscosityof the matrix and induce pressure differentials that allow the matrixresin to flow. The resin flow may fill the void spaces within theprepreg layup and displace gases from the layup when the viscosity ofthe matrix is sufficiently low in order to facilitate consolidation.Consequently, the layup is cured at a more elevated temperature withinthe same autoclave or oven to produce a final composite part.

A composite sandwich structure may be produced using the porous prepregsdisclosed herein. In one embodiment, a center core 130 composed of wood,foam, honeycomb, or other structural materials is sandwiched between twoprepreg layups 131, 132, as shown in FIG. 13, wherein some or all of theprepreg plies in the layup contain surface openings. The resultingcomposite sandwich structure is illustrated by FIG. 14. Optionally,doubler layers may be placed between porous prepreg plies so as tocreate elongated reinforcing regions. Furthermore, unimpregnated, orpartially impregnated lightweight scrims, such as fiberglass, carbon,thermoplastic or other materials woven or unwoven, may be introducedwithin the layups in selected localities in order to facilitate theremoval of gases or to increase mechanical properties such as damagetolerance.

When the prepreg layup incorporates a core structure, an adhesivematerial may also be employed in order to bond the core to the prepregmaterial prior to curing of the prepreg layup. As open center corestructures, such as honeycomb structure, may contain a significantamount of gases, the adhesive layer may also be perforated in order tofacilitate removal of the gases.

The thermal treatment disclosed herein may be incorporated into thecomposite part manufacturing process at a parts builder, either beforeor during layup. The heat treatment of any prepreg ply could be carriedout in-situ during the prepreg layup process by applying heat to aprepreg material or ply before it is laid down, as it is being laiddown, or after a prepreg ply has been laid down, and prior to theplacement of a subsequent next ply. For example, the process mayinclude: laying down a prepreg ply covered on one surface with a releasepaper or polyester film; heat treatment using a heated roller, hot airwand, hot iron, etc., to form surface openings; removing the releasepaper/polyester film; laying down the next prepreg ply; and repeated asnecessary until a prepreg layup of desired thickness is formed.

EXAMPLES

The following examples are provided to demonstrate the benefits of theembodiments of the disclosed curable prepregs. These examples arediscussed for illustrative purposes and should not be construed to limitthe scope of the disclosed embodiments.

Example 1

A prepreg fabric was prepared by a hot-melt process using a prepreggingmachine, in which two resin films formed from a toughened epoxy-basedresin, Cycom 5320 (available from Cytec Industries Inc.) are pressedagainst the top and bottom surfaces of an 8-Harness satin weave,carbon-fiber fabric, whereby the fabric is sandwiched between the tworesin films. Each resin film was formed on a silicone coated releasepaper and has an areal unit weight of 106 gsm per film. The carbon-fiberfabric has a FAW of (370) gsm and thickness of 0.0146 in. Heat andpressure were applied to the laminate to cause the resin films to meltand penetrate partially through the thickness of the fabric. One of therelease papers was replaced with a smooth polyester film afterprepregging to facilitate winding onto a roll. The pre-impregnatedprepreg with the release paper on top and polyester film underneath washeated in an oven for 2-5 minutes at 200° F. (93° C.). This heat cycletime has been found to be sufficient for opening up the enclosed airpockets without impacting the mechanical or physical characteristics ofthe prepreg. FIGS. 15-17 show the top-view image of the prepreg surface,with the release paper removed, at 1 minute, 4 minutes and 7.5 minutes,respectively, of heating time. The openings coincided with thecrossover/under locations (i.e. up/down tow portions) in the satin weavefabric. As can be seen from FIGS. 15-17, the sizes of the openingsbecame larger over time. After 7.5 minutes, some openings, which werealigned in the same row and adjacent to crossover/under locations of theadjacent tows were touching each other, as can be seen from the image ofFIG. 13. These openings correspond to the up/down tow portions in thesatin weave fabric (as indicated by reference number 70 in FIG. 7). Itwas noted that the openings were formed in only the release paper sideof the heat-treated prepreg.

Example 2

A prepreg fabric was prepared by using the prepregging system depictedin FIG. 12. The fabric used was a plain weave carbon-fiber fabric, andthe resin films applied to opposite sides of the fabric were formed fromCycom 5320 epoxy-based resin. Each resin film was formed on a siliconecoated release paper and has an areal unit weight of 55 gsm. Thecarbon-fiber fabric has a FAW of 190 gsm and thickness of 0.0083″. Forpartial impregnation, 20 psi was applied at the first nip; 220° F. (104°C.) was the temperature at the heating plate; and a gap of less than 0.5in (12.7 mm) was provided at the second nip to limit compaction force.FIG. 18 shows the top-view image of the heat-treated prepreg surfacewith the release paper removed. The openings in the prepreg surfacecoincide with the interstices in the plain weave fabric. Furthermore, itwas noted that the openings were formed in both major surfaces of theheat-treated prepreg.

Example 3

For comparison, a control prepreg was prepared as described in Example 1without the thermal post-treatment for creating surface openings. FIG.19 shows the resulting prepreg surface with enclosed air bubbles formedbeneath the continuous resin film. These air bubbles correspond to theup/down tow portions in the satin weave fabric. Thus, it can be seenthat, without the heat treatment before curing, entrapped air from airpockets, and air in between plies that cannot escape remained in theresulting cured prepreg due to the fact that the continuous resin filmlimits the removal of air from the prepreg.

Example 4

A 12″×12″ monolithic panel consisting of 15 plies of 5320/8HS prepregmaterial as described in Example 1 was constructed and cured. Forcomparison, the same panel was constructed using untreated 5320/8HSprepreg material and cured under the same conditions. The resultingporosity was reduced from 1.31% without treatment to 0.04% withheat-treatment. FIG. 20 shows the cross-section of the panel consistedof material without treatment, and FIG. 21 shows the cross-section ofthe panel consisted of heat-treated material.

Example 5

A honeycomb core sandwich structure was assembled based on theconfiguration shown FIG. 13, wherein 10 porous prepreg plies (201) wereplaced over a honeycomb core (202) and 14 porous prepreg plies (203)were placed under the honeycomb core. The porous prepreg plies wereproduced by partially impregnating satin weave, carbon fiber fabric withCycom 5320 resin, followed by thermal treatment to create surfaceopenings as described in Example 1. The assembled sandwich structure wasvacuum bagged, consolidated at room temperature and cured in an oven(not autoclave).

For comparison, a standard honeycomb core sandwich structure wasassembled, consolidated and cured in the same manner except that theprepreg plies were not thermally treated to create surface openings.

Porosity was measured at different sections of the cured product,including flange, bevel sections, central core, and an average porositywas calculated. Porosity was measured by visual microscopy of polishedpanel cross sections

The cured product resulting from using porous prepregs was found tocontain about 0.05% porosity on average, as compared to 2.0% porosityfor the cured, standard product.

Example 6

A honeycomb core sandwich structure was assembled using porous prepregplies and was based on the configuration of FIG. 13. The porous prepregplies used for this structure were composed of plain weave, carbon fiberfabric and Cycom 5320 resin, and the openings in the prepreg plies wereproduced by in-situ heating during the prepregging process as describedin Example 2. Subsequently, the assembled sandwich structure was vacuumbagged, consolidated at room temperature and cured in an oven (notautoclave). For comparison, a standard honeycomb core sandwich structurewas assembled, consolidated and cured in the same manner except that theprepreg plies were not thermally treated to create surface openings.

The cured product resulting from using porous prepregs was found tocontain about 0.18% porosity on average, as compared to 1.74% porosityfor the cured, standard product.

Although the foregoing description has shown, described, and pointed outthe fundamental novel features of the present teachings, it will beunderstood that various omissions, substitutions, and changes in theform of the detail of the apparatus as illustrated, as well as the usesthereof, may be made by those skilled in the art, without departing fromthe scope of the present teachings. Consequently, the scope of thepresent teachings should not be limited to the foregoing discussion, butshould be defined by the appended claims.

The terms “approximately”, “about” and “substantially” as used hereinrepresent an amount close to the stated amount that still performs thedesired function or achieves the desired result. For example, the terms“approximately”, “about”, and “substantially” may refer to an amountthat is within less than 10% of, within less than 5% of, within lessthan 1% of, within less than 0.1% of, and within less than 0.01% of thestated amount.

What is claimed is:
 1. A curable prepreg comprising: a woven fabrichaving two opposing faces and a weaving pattern in which one or moretows in a first weaving direction float over one or more tows in asecond weaving direction, then pass under one or more tows in the secondweaving direction; wherein a pocket is defined in the fabric surfacewhen a first tow in the first weaving direction passes over a second towin the second weaving direction then under an adjacent third tow in thesecond weaving direction, or when the first tow passes under a secondtow in the second weaving direction then over an adjacent third tow inthe second weaving direction, and the pocket location is at a crossoverunder location in the weaving pattern, wherein the crossover/underlocation is defined by the portion of the first tow that is going up orgoing, down between the adjacent second and third tows a curable,hot-melt resin film substantially covering one or both face(s) of thefabric and penetrating partially through the thickness of the fabricleaving an inner section of the fabric, in the thickness direction,substantially free of the resin film; and an array of openings in theone or both resin film(s), each opening exposing a single pocket in thefabric surface at a single crossover/under location and configured tocreate a fluid flow path from the inner section of the fabric to atleast one outer surface of the prepreg, or from at least one outersurface of the prepreg to the inner section, or from one outer surfaceof the prepreg to an opposite outer surface, or combination thereof. 2.The curable prepreg of claim 1, wherein the weaving pattern of the wovenfabric is satin weave or twill weave.
 3. The curable prepreg of claim 1,wherein the weaving pattern is satin weave, and the openings are formedthrough one or both surfaces of the prepreg.
 4. The curable prepreg ofclaim 1, wherein the hot-melt resin film is substantially solid at atemperature within the range of 20° C.−25° C. and becomes flowable at anelevated temperature below an onset curing temperature of the resinfilm.
 5. The curable prepreg of claim 1, wherein the hot-melt resin filmcomprises one or more thermoset resins as a major component, and issubstantially free of any organic solvent.
 6. The curable prepreg ofclaim 1, wherein the hot-melt resin film comprises one or more epoxyresins, a curing agent, and at least one thermoplastic or elastomericcompound.
 7. The curable prepreg of claim 1, wherein each tow comprisesa plurality of fiber filaments, which are formed of a material selectedfrom: glass, carbon, aramid, polyethylene (PE), boron, quartz, basalt,ceramic, polyester, poly-p-phenylene-benzobisoxazole (PBO), andcombinations thereof.
 8. The curable prepreg of claim 1, wherein theopenings are substantially circular in cross-section with diameterwithin the range of 0.1-3 mm.
 9. The curable prepreg of claim 1, whereinthe openings are approximately rectangular in cross-section with widthand length within the range of 0.1 mm-3 mm.
 10. A curable prepreg withan array of surface openings formed by a method comprising: (a)partially impregnating a woven fabric with a curable, hot-melt resinsuch that a continuous resin film covers each face of the fabric andpenetrates partially through the thickness of the fabric leaving aninner section of the fabric, in the thickness direction, substantiallyfree of said resin, wherein the woven fabric has two opposing faces anda weaving pattern in which one or more tows in a first weaving directionfloat over one or more tows in a second weaving direction, then passunder one or more tows in the second weaving direction, wherein a pocketis defined in a face of the fabric when a first tow in the first weavingdirection passes over a second tow in the second weaving direction thenunder an adjacent third tow in the second weaving direction, or when thefirst tow passes under a second tow in the second weaving direction thenover an adjacent third tow, and the pocket location is at acrossover/under location in the weaving pattern, wherein thecrossover/under location is defined by the portion of the first tow thatis going up or going down between the adjacent second and third tows,and wherein the partially impregnated fabric comprises a plurality ofenclosed air pockets, each air pocket coincides with a pocket in thefabric; and (b) heating the partially impregnated fabric so that theresin film on at least one face of the fabric becomes flowable, andsubsequently, portions of the resin film over the air pockets breakopen, thereby creating openings in the resin film that correspond tolocations of the air pockets, wherein each opening exposes a singlepocket in the face of the fabric at a single crossover/under location,wherein the openings are configured to provide fluid flow paths from theinner section of the fabric to at least one outer surface of theprepreg, or from at least one outer surface of the prepreg to the innersection, or from one outer surface of the prepreg to an opposite outersurface, or combination thereof.
 11. The curable prepreg of claim 10,wherein the weaving pattern of the woven fabric is satin weave or twillweave.
 12. The curable prepreg of claim 10, wherein the weaving patternis satin weave, and the openings are formed through one or both surfacesof the prepreg.
 13. A method for fabricating a curable compositematerial with an array of surface openings, the method comprising: (a)partially impregnating a woven fabric with a curable, hot-melt resinsuch that a continuous resin film covers each face of the fabric andpenetrates partially through the thickness of the fabric leaving aninner section of the fabric, in the thickness direction, substantiallyfree of said resin, wherein the woven fabric has two opposing faces anda weaving pattern in which one or more tows in a first weaving directionpass over one or more tows in a second weaving direction, then passunder one or more tows in the second weaving direction wherein a pocketis defined on a face of the fabric when a first tow in the first weavingdirection passes a second tow in the second weaving direction then underan adjacent third tow in the second weaving direction, or when the firsttow passes under a second tow in the second weaving direction then overan adjacent third tow, and the pocket location is at a crossover/underlocation in the weaving pattern, wherein the crossover/under location isdefined by the portion of the first tow that is going up or going downbetween the adjacent second and third tows, and wherein the partiallyimpregnated fabric comprises a plurality of enclosed air pockets, eachair pocket coincides with a pocket in the fabric surface; and (b)heating the partially impregnated fabric so that the resin film on atleast one face of the fabric becomes flowable, and subsequently,portions of the resin film over the air pockets break open, therebycreating openings in the resin film that correspond to locations of theexpanded air pockets, wherein each opening exposes a single pocket inthe face of the fabric at a single crossover/under location, wherein theopenings are configured to provide fluid flow paths from the innersection of the fabric to at least one outer surface of the compositematerial or from at least one outer surface of the composite material tothe inner section, or from one outer surface of the composite materialto an opposite surface, or combination thereof.
 14. The method of claim13, wherein the outer surface of the resin film is covered with arelease paper or polyester film during heating.
 15. A method forfabricating a curable composite part comprising: laying down a pluralityof prepreg plies to form a prepreg layup, wherein at least some prepregplies are porous prepreg plies with surface openings, and each porousprepreg ply is the composite material formed by the method according toclaim 13.